Recently, many Earth-sized planets have been discovered around stars other than the Sun that might possess appropriate conditions for life. The development of theoretical methods for assessing the putative habitability of these worlds is of paramount importance, since it serves the dual purpose of identifying and quantifying what types of biosignatures may exist and determining the selection of optimal target stars and planets for subsequent observations. This Colloquium discusses how a multitude of physical factors act in tandem to regulate the propensity of worlds for hosting detectable biospheres. The focus is primarily on planets around low-mass stars, as they are most readily accessible to searches for biosignatures. This Colloquium outlines how factors such as stellar winds, the availability of ultraviolet and visible light, the surface water and land fractions, stellar flares, and associated phenomena place potential constraints on the evolution of life on these planets.

• Received 1 October 2018

DOI:https://doi.org/10.1103/RevModPhys.91.021002